Injection of gases into liquids is used in a wide range of chemical and metallurgical applications. For example, oxygen gas is injected into water to dissolve oxygen for fish farming or for treatment of aerobic treatment waste water. Argon or nitrogen gas is injected into molten aluminum for refining to remove or to “strip” dissolved hydrogen. Oxygen gas is injected into molten steel for refining to react and remove dissolved carbon.
Gas bubbles can be formed in a liquid by injecting gas from a small nozzle submerged in the liquid. When the gas flow rate is very low, a series of small bubbles is generally produced which are of uniform size and well separated. As the gas flow rate is increased, more bubbles are formed per unit time and coalescing of bubbles can start to occur. As used herein, “coalescing” means the joining together of two or more bubbles to form one bubble, with or without the associated formation of one or more smaller satellite bubbles as well; and bubbles are considered “non-coalescing” if they do not join together but instead remain separated in the liquid. In general, when the gas flow rate through the submerged nozzle is increased further, more frequent coalescing occurs (for instance, four bubbles joining together to form one larger bubble). As the gas flow rate is increased even more, chaotic bubble forming behavior is observed in which many bubbles of various sizes and shapes are formed in a turbulent fashion.
In most of these applications it is desirable to create a large number of small gas bubbles well dispersed in the liquid to accelerate the mass transfer process. Many technologies have been developed to generate small bubbles in liquids. They include injection of gas from a small submerged nozzle at a high velocity, mechanical stirring of a gas-liquid mixture to break up large gas bubbles, and injection of gas from a narrow space between a rotating disc and a stationary disc. In these processes, the viscosity of the liquid is relatively low and the residual gas bubbles rise to the liquid surface quickly and are removed without being trapped in the final product.
In the commercial-scale manufacture of glass articles such as bottles, window glass, and the like, solid feed material is melted in large furnaces. The feed material can comprise pieces of glass typically comprising recycled glass articles, and/or the conventional raw materials of glass manufacture, e.g. sand, limestone and soda ash.
The solid glassmaking feed material enters one end or feed port of the glassmelting furnace, where the feed material is exposed to temperatures in excess of 1300C. to melt the feed materials. The resulting molten glass is withdrawn from an exit port of the glassmelting furnace and conveyed to forming stations where it can be formed into the desired useful articles. The molten glass that is withdrawn from the glassmelting furnace can optionally be conveyed to a holding tank, where it is held in the molten state, before it is conveyed to the forming stations.
The molten glass that is formed in these steps typically contains a large number of small bubbles, called “seeds”. Since these seeds are rarely desirable in the final, solid glass product, it is highly desirable to remove them from the glass, and to do so while the glass is still molten. The process of removing small bubbles from molten glass is typically called “fining”. The small bubbles can rise to the surface of the molten glass on their own, but only over a period of time that is usually too long to be economically acceptable to the operator. Accordingly, it is necessary to employ other methods to accelerate the removal of these undesired bubbles from the molten glass. Chemical fining agents such as sodium sulfate and antimony oxide are commonly used to generate fining gases, upon dissociation at high temperatures, which diffuse to the small defect bubbles, or seeds, and grow them in size so that they can rise to the molten glass surface faster.
Gas bubblers are often used in glass melting furnaces in order to enhance convective current of molten glass. They are typically located in the floor of the glass tank and the rising gas bubbles lift viscous molten glass and enhance the flow of molten glass. U.S. Pat. No. 2,890,548 teaches methods and apparatus for controlling convective currents of molten glass by injecting gas through a bubbling nozzle. It is advantageous to produce large bubbles to enhance the convective current. Pulsed injection of high pressure gas is taught to increase the bubble size. In U.S. Pat. No. 3,874,865 a self-controlling gaseous bubbler system to produce multiple bubbles from multiple injector nozzles is described using high and low pressure gas supply lines and pneumatic logic elements.
A recent advancement in the art of glass fining is to inject helium gas into molten glass to accelerate fining. Some of the helium gas diffuses out from the helium bubbles, migrates through the glassmelt and into seeds and accelerates the growth of the seeds. While helium gas diffuses out of the helium bubbles, other gases dissolved in the molten glass, such as nitrogen, carbon dioxides, oxygen and sulfur dioxides, diffuse into the helium bubbles. Thus the helium bubbles accelerate the growth of seeds and at the same time “strip” out other undesirable gases dissolved in the molten glass. When the initial size of the helium bubble is large, say greater than 4 cm in diameter, the bubble rises quickly to the surface and only a small fraction of helium gas contained in the bubble can diffuse into seeds and promote the fining action. Most of the helium gas is thus wasted. When the initial size of the helium bubble is too small, say less than 0.5 cm, there is a risk of creating a small defect bubble of helium in the glass product as the size of the helium bubble shrinks with helium gas diffusion and the velocity of the bubble rise becomes too slow.
It has been determined by the present inventor that the passage of bubbles upwards through the molten glass from the bottom of the tank is significantly more effective at removing the undesired small bubbles if the bubbles which are introduced at or through the bottom of the tank are relatively uniform in diameter, since bubbles having too large a variation in size will flow upwards through the molten glass at varying rates which are a function of the size of the respective bubbles, in which event the bubbles can tend to collide and coalesce with each other leading to unsatisfactory performance especially in the removal and the rate of removal of the small bubbles. In addition, it has been determined that bubbles having a uniform diameter which is in the range of 0.5 to 4 cm, preferably in the range of 0.5 to 2 cm, perform advantageously in the removal of the undesired small bubbles.
Although the prior art has taught the use of pulsed injection of high pressure gas into multiple injectors to make large bubbles, the injection system is complex and expensive. There remains a need in this field for a reliable technique for forming bubbles from the bottom of the tank containing the molten glass, wherein the diameters of the formed bubbles are uniformly within a narrow range of diameters falling within the range of 0.5 to 4 cm in diameter.